OU bug 10375

It looks like a trivial fix, but I think I want to also change the
code logic because it looks cr** to me (it's doing If-Modified-Since checks
with an ==, surely it should be a <= really?)

I haven't been able to reproduce this one because browsers don't tend to send
the header, but I can see how it happens; basically if you have a blog with no
entries (or no comments, for the comment-only feed) then this error will occur
if the reader sends the header.

I also note that the HTTP response is kind of screwed - at present our RSS
feeds all send the default Moodle 'never ever cache this' headers, which could
potentially have an impact on performance, I'd think!

The behaviour here occurred only if not in DEBUG_DEVELOPER mode. I decided this
was not really necessary (if you're in developer mode and you really really
want to not see a cached rss feed, just press reload in your browser) and
potentially dangerous (this code never runs on a dev server, which is one
reason, admittedly among many, why nobody noticed it never worked).